Silicon or silicon-on-insulator (SOI) substrates are commonly used for the production of microelectronic devices. As is well known per se, such a substrate includes a useful layer of silicon and a buried oxide, arranged on a supporting substrate. In order to allow even greater integration of transistors on the substrates, the reduction of the lateral dimensions of the elementary components and the thinness of the etching require substrates with increasing quality, both in terms of crystalline quality and layer uniformity and in terms of specific contamination.
In order to produce an SOI substrate, high-temperature thermal treatments are applied, in particular during the steps of consolidating the bonding interface or the finish of the useful layers of silicon and buried oxide. Thermal treatment systems, in particular vertical furnaces, capable of treating a plurality of substrates simultaneously, are particularly suited to these types of treatments. As shown in FIG. 1A, a vertical furnace 1 is made up mainly of a chamber 2 (or tube) inside of which a loading column 3 supporting the plurality of substrates 10 can perform a vertical translation movement in order to load/unload the plurality of substrates 10 and to keep them in the chamber 2. Heating elements 4, arranged around the chamber 2, as well as at least one intake path 5 for new gas and an outlet path 6 for gas to be discharged, also make up this type of furnace.
As an example, SOI substrates may be subjected to a high-temperature thermal treatment (>1,100° C.) under an inert atmosphere in order to smooth the surface of the silicon and/or to dissolve all or part of the buried oxide (referred to as BOX). Under these treatment conditions, a reduction of the thickness of the BOX is observed due to a dissolution phenomenon. Gaseous silicon monoxide (SiO) is a product of the dissolution reaction. The silicon monoxide escapes from the surface of the SOI substrates in a quantity proportional to the dissolution speed, before being carried by the stream of heat-transfer gas circulating in the chamber 2 of the vertical furnace 1, towards the outlet path 6 of the furnace (also referred to as exhaust) located at the bottom of the chamber 2. The dissolution phenomenon is notably reported in the document “Novel trends in SOI Technology for CMOS applications” by O. Kononchuck et al., published in Solid State Phenomena, volume 156-158 (2010), pages 69 to 76.
In the bottom of the chamber 2, the gas cools gradually when it leaves the area close to the heating elements 4 and circulates towards the exhaust 6. Since the SiO is not highly soluble in the heat-transfer gas, it condenses below a critical temperature on the solid portions of the bottom of the furnace 1: mainly, the inner walls of the chamber 2, the ducts of the outlet path 6 and the lower parts of the loading column 3 of the substrates 10 (FIG. 1B). SiO deposit 7 thickens throughout the treatment of the substrates 10 and can reach a thickness of several micrometers. Made up of a mixture of SiO, Si and SiO2, it is under considerable stress and delaminates in the form of shavings when it becomes too thick. During the loading and unloading phases of the furnace 1, the loading column 3 enters and exits the chamber 2 and the substrates 10 then pass directly into the bottom of the chamber 2 where the SiO deposit 7 is present (FIG. 1C). The delamination of the latter induces specific contamination of the substrates 10, which is particularly detrimental to the final quality thereof.
Quartz protective screens can be placed against the bottom of the chamber 2, so as to collect most of the SiO deposit 7, thus ensuring the integrity of the chamber 2 of the furnace 1. These screens are sacrificial: they can be changed and make it possible to extend considerably the useful life of the chambers 2, which are extremely expensive parts of a thermal treatment system that take long to change.
Nevertheless, unless they are changed very regularly, which is not economically feasible, these protective screens are not a solution to the problem of specific contamination of the substrates 10 by the delamination of SiO deposits, since the loading column 3 of the substrates 10 continues to pass near contaminated screens during the loading and unloading steps.